The lead-acid battery is the oldest and most popular type of rechargeable energy storage device, dating back to the late 1850's when initially conceived by Raymond Gaston Plante. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, the lead-acid battery can supply high-surge currents, allowing the cells to maintain a relatively large power-to-weight ratio. These features, along with their low cost, make lead-acid batteries attractive for use in motor vehicles, which require a high current for starter motors. A lead-acid battery is generally composed of a positive electrode and a negative electrode in an electrolyte bath. Typically, the electrodes are isolated by a porous separator whose primary role is to eliminate all contact between the electrodes while keeping them within a minimal distance (e.g., a few millimeters) of each other. A separator prevents electrode short-circuits by containing dendrites (puncture resistance) and reducing the Pb deposits in the bottom of the battery.
A fully charged, positive lead-acid battery electrode is typically lead dioxide (PbO2). The negative current collector is lead (Pb) metal and electrolyte is sulfuric acid (H2SO4). Sulfuric acid is a strong acid that typically dissociates into ions prior to being added to the battery:H2SO4→H++HSO4−
As indicated in the following two half-cell reactions, when this cell discharges, lead metal in the negative plate reacts with sulphuric acid to form lead sulphate (PbSO4), which is then deposited on the surface of the negative plate.Pb(s)+HSO4−(aq)→PbSO4(s)+H+(aq)+2e− (negative-plate half reaction)PbO2(s)+3H+(aq)+HSO4−(aq)+2e−→PbSO4(s)+2H2O (positive-plate half reaction)
During the discharge operation, acid is consumed and water is produced; during the charge operation, water is consumed and acid is produced. Adding the two discharge half-cell reactions yields the full-cell discharge reaction:Pb+PbO2+2H2SO4→2PbSO4+2H2O (full-cell discharge equation)
When the lead-acid battery is under load, an electric field in the electrolyte causes negative ions (in this case bisulfate) to drift toward the negative plate. The negative ion is consumed by reacting with the plate. The reaction also produces a positive ion (proton) that drifts away under the influence of the field, leaving two electrons behind in the plate to be delivered to the terminal.
Upon recharging the battery, PbSO4 is converted back to Pb by dissolving lead sulphate crystals (PbSO4) into the electrolyte. Adding the two charge half-cell reactions yields the full-cell charge reaction.PbSO4(s)+H+(aq)+2e−→Pb(s)+HSO4−(aq) (negative-plate half reaction)PbSO4(s)+2H2O→PbO2(s)+3H+(aq)+HSO4−(aq)+2e− (positive-plate half reaction)PbSO4(s)+H+(aq)+2e−→Pb(s)+HSO4−(aq) (full-cell charge equation)
When the battery repeatedly cycles between charging and discharging, the efficiency of dissolution of PbSO4 and conversion to Pb metal decreases over time. As a result, the amount of PbSO4 continues to increase on the surface of negative plate and over time forms an impermeable layer of PbSO4, thus restricting access of electrolyte to the electrode.
Over the years, several additives, including expanders, have been used in an attempt to lessen the growth of lead sulphate and improve battery performance. Expanders act as anti-shrinkage agents and are an important component of lead/acid batteries because they prevent performance losses in negative plates that would otherwise be caused by passivation and structural changes in the active material. To make a negative plate spongy and prevent the solidification of lead, expanders were developed from a mixture of carbon black, lignin derivatives (e.g., lignosulphate, lignosulfonates), and barium sulphate (BaSO4). These expanders can be incorporated into a battery's negative plates in several ways, including adding the individual components to a paste mix and adding a pre-blended formulation.
Carbon black is typically added to the negative active material (NAM) to increase: (i) electrical conductivity; (ii) surface area of the NAM; and (iii) nucleating PbSO4 crystals. Carbon black is substantially pure elemental carbon, typically in the form of colloidal particles produced by an incomplete combustion or thermal decomposition of gaseous or liquid hydrocarbons under controlled conditions. It is a black, finely divided, pellet or powder.
The presence of lignin derivatives (e.g., lignosulphate, lignosulfonates, and other complex aromatic polyethers) helps prevent the formation of an obstructive PbSO4 layer on the electrode surface and facilitates the formation of a porous layer built up of individual PbSO4 crystals. Lignin derivatives have the property of being strong antiflocculents (e.g., they prevent colloids from coming out of suspension in the form of flocs or flakes) and comprise a large hydrophobic organic part (R+) and a small hydrophilic inorganic fraction (SO3−). As a result, lignin derivatives are water-soluble. For example:RS03Na→RSO3−+Na+
The hydrophobic part of the RSO3− anion will be adsorbed on the surface of the lead particles, and thus the hydrophilic part of the anion will phase-out to the aqueous electrolyte phase. This results in an increase in the repulsion potential, which prevents the particles from coalescing or sintering. Many different lignosulfonates have been used in expanders; however, their effects on the performance of lead/acid batteries varies dramatically based on the expander formula and battery type application (e.g., Starting, Motive, Standby).
Barium sulfate, being isomorphic to PbSO4, acts as a nucleation agent and ensures uniform distribution of PbSO4 crystals throughout the active mass volume. The ability of barium sulfate to act as a site for lead sulfate precipitation is due to the similar structures of the two compounds. Strontium sulfate has also been shown to be an effective expander. The inert barium sulfate provides a large number of sites for the precipitation of lead sulfate crystallites and thereby prevents its deposition as a thin, impermeable, passivating PbSO4 film.
A notable difference between expanders used in automotive and industrial applications is the ratio of barium sulfate to carbon. For example, in automotive batteries, a high fraction of lignosulfonate (25-40%) may be used, whereas in industrial batteries, a smaller percentage of lignosulfonate may be used (0-10%). The higher percentage of lignosulfonate in automotive plates may be useful in producing high cold-cranking amperes, whereas a larger amount of barium sulfate in industrial plates may help prevent passivation during deep cycling and provide greater durability.
Conductive additives in positive plates, such as BaPbO3 (Barium metaplumbate); Pb3O4 (Red lead), Titanium based compounds (e.g., Ti4O7, TiSi2, TiO2), and graphite have been used to improve the power density and corrosion resistance in lead-acid batteries. Similarly, higher surface area additives (e.g., glass microspheres, particulate silica, zeolite, and porous carbons) have also been added to negative paste to improve electrolyte access and enhance cycle life.
For several decades, carbon has been a popular additive to the NAM in lead-acid batteries. Although the role of carbon in NAM may be generally unclear, several beneficial effects have been identified. For example, carbon nucleates the PbSO4 crystals, resulting in smaller crystals that may be more easily dissolved into the electrolyte during charging processes. This restricts the progress of plate sulfation (e.g., formation of a PbSO4 layer) and increases the useful life of the battery in high-rate, partial state-of-charge (HRPSoC) duty. High surface-area carbons can act as a reservoir for electrolyte within NAM, thus reducing the possibility of plate dry-out.
A combination of highly conductive graphite, high surface area carbon black and/or activated carbon is often used in NAM. In graphite, the atoms are typically arranged in planar hexagonal networks (e.g., graphene layers) held together by strong sp2 bonds resulting in high in-planar electronic conductivity. A disordered carbon typically arises when a fraction of carbon atoms is sp3 hybridized rather than sp2. The electronic conductivity of mixed carbon depends on the distribution of sp3 carbon in sp2 domains. Although graphite additives in active material decrease the resistivity of the paste by forming a conductive path around the PbSO4 crystals, they are reported to have lower surface areas (typically in the order of 10-30 m2/g). A second carbon additive is generally required to increase the total surface area of the NAM to improve the accessibility of electrolyte. Carbon blacks and activated carbons with surface areas between 200-2000 m2/g may be added in conjunction with graphite to improve surface area as well as electronic conductivity. Activated carbon is a form of carbon that has been processed to greatly increase porosity, thus greatly increasing its surface area (e.g., 1 gram of activated carbon may have surface area in excess of 500 m2).
Numerous attempts have been made to overcome the above-mentioned problems. For example, U.S. Pat. No. 6,548,211 to Kamada, et al., discloses the addition of graphite powder having a mean particle size smaller than 30 μm added in the range of about 0.3% to 2% by weight. U.S. Patent Publication No. 2010/0015531 to Dickinson, et al., discloses a paste for negative plate of lead acid battery having a activated carbon additive loadings of 1.0% to 2.0% by weight. The activated carbon additive, taught by Dickinson, has a mesopore volume of greater than about 0.1 cm3/g and a mesopore size range of about 20-320 angstroms (Å) as determined by the DFT nitrogen adsorption method. U.S. Patent Publication No. 2010/0040950 to Buiel, et al. describes a negative electrode having a mixture of activated carbon (˜5-95% by weight), lead (5-95% by weight), and conductive carbon (5-20% by weight). U.S. Pat. No. 5,547,783 to Funato, et al., describes various additives, including carbon, acetylene black, polyaniline, tin powder, and tin compound powder having an average particle diameter of 100 μm or less. U.S. Pat. No. 5,156,935 to Hohjo, et al., describes electro-conductive whiskers made of carbon, graphite or potassium titanate—useful as additives for the negative plate of a lead-acid battery—having a diameter of 10 μm or less, aspect ratio of 50 or more, and a specific surface area of 2 m2/g(21). Unfortunately, none of these previous attempts have been able to achieve the benefit of both higher surface area and higher electronic conductivity in a single carbon material.
Carbon blacks and activated carbons have the ability to accept a higher charge because of their higher surface areas and enhanced electrolyte accessibility. Unfortunately, because of their porous structures, carbon blacks and activated carbons have poor retention on particle size during paste mixing and cycling. As a result, carbon blacks and activated carbons often disintegrate, causing the carbon to bleed out of the plate over period of time, resulting in active material shedding from the grids.
Graphites, by contrast, with ordered structures, are advantageous because they are both inert to electrochemical reactions during charge-discharge cycles and resist disintegration during cycle life tests over an extended period. Unfortunately, graphites have lower surface areas, thus restricting electrolyte access and resulting in an active material with lower charge acceptance.
Despite the numerous existing battery additives, there is a need for an improved battery additive that (i) is inert to electrochemical reactions during charge-discharge cycles; (ii) resists disintegration during cycle life tests over an extended period; and (iii) yields an increased charge acceptance.